1. Field of the Invention
The present invention relates generally to a feed-forward control for power supplies used in battery chargers, and relates more particularly to a dynamic feed back control adaptive to transient events.
2. Description of the Related Art
Battery chargers for various applications including portable personal computers have been developed over the course of a number of years to improve performance characteristics while reducing size and complexity. Battery chargers are typically connected to an input power supply also used in powering an application such as a portable PC. In conventional topologies in portable end equipment, such as a portable PC, the battery charger is located in the end equipment and draws its power from an AC/DC power converter. The AC/DC converter supplies a consistent, fixed voltage to the battery charger, in addition to supplying power to the application. In this traditional topology, a power converter such as a DC-DC buck converter, located in the end equipment, is used to implement a battery charger function. In such topology control circuits, the power converter usually monitors specific system and battery pack parameters and adjusts a power converter duty cycle to obtain a desired charge current and charge voltage output. In such a DC-DC power converter, a conventional practice is to use the input voltage in a feed forward function, so that the overall loop response is optimized with respect to input line transients. The feed forward enables implementation of a higher bandwidth control loop, when compared to topologies that do not use a feed forward function.
In more recent developments, additional topologies have been developed that provide for the battery charger power stage function to be implemented by the power conversion stage already existent in the AC-DC converter/adapter. In this new topology the adapter output voltage is adjusted to obtain a desired charge current and charge voltage output. The AC-DC adapter output voltage is set as defined by an error signal sent to the adapter from the battery charge controller located in the end equipment. The charge current and charge voltage are directly proportional to the AC-DC adapter output voltage, since the relationship between the charge parameters and the adapter output is determined by ohmic drops in the path from the adapter to the battery charger. Some of the drops include sense resistors and on/off switches as indicated in FIG. 1.
In this new topology, the AC-DC adapter output voltage is adjusted to supply the system current and the battery charge current. As a result, the AC-DC adapter output will be dynamically adjusted, being increased or decreased by the control circuit, located in the end equipment, to deliver power to the system while charging the battery pack, under distinct system operating conditions and battery voltage ranges.
Locating the charger power stage in the AC-DC adapter reduces the power dissipation in the end equipment. In addition, the new configuration limits the voltage supplied by the AC-DC adapter to a value close to the battery voltage range plus ohmic drops, effectively enabling optimization of other end equipment power converter stages and improving overall system efficiency.
In the traditional topology, with a DC-DC converter in the end equipment, the AC-DC adapter voltage has a fixed value output. The DC-DC converter uses a traditional feed forward concept to adjust the DC-DC converter duty cycle based on input voltage variations, enabling a faster loop response. However, with the more recent adapter output voltage control topology, with a power converter in the AC-DC adapter, a traditional feed forward circuit presents several difficulties, as the AC-DC adapter voltage dynamically changes under distinct system operating conditions as defined by the control loop. The dynamic change in conjunction with the traditional feed forward circuit may generate oscillatory conditions. For instance, if the control loop is requesting an AC-DC adapter voltage output increase, i.e., an increase in duty cycle, a traditional feed forward circuit in the end equipment would prompt the control circuit to reduce the duty cycle, i.e., decrease output voltage.
Completely removing the feed forward function from the end equipment is not a viable option. Usually the AC-DC adapter power stage is implemented in a fly-back configuration, which has a very low overall loop bandwidth to be stable under distinct load conditions. As a result, removing a feed forward function produces a very slow response to system load transients, potentially resulting in long transient times with a potential for excessive charge currents being applied to the battery pack.
It would be desirable to obtain a battery charge controller circuit with a feed forward control that does not create oscillation in the output voltage. It would also be desirable to obtain a simplified means of controlling loop bandwidth and response time.